The present invention relates generally to a hybrid electric vehicle and more particularly, to an apparatus and method for regulating the power of a vehicle high-voltage bus.
The development of alternative energy sources for use in automotive vehicles is a goal of automobile manufacturers. The alternative energy sources must provide the required power needed to operate a vehicle and at the same time be energy efficient, reduce emissions, and be cost effective. One such alternative energy source under consideration is a fuel cell energy system. Fuel cell technology is continuously being improved for use in Electric Vehicles (EVs). A fuel cell is a very efficient power source, which is capable of providing the power needs required for an automotive vehicle. The fuel cell also produces zero emissions.
EVs have an electric propulsion system. An electric propulsion system has a fuel cell, an electric motor/generator, and a battery. The fuel cell creates electric energy, which is either stored in the battery or used in propelling the automotive vehicle. The electric energy created by the fuel cell powers the electric motor/generator in turn propelling the automotive vehicle. The electric motor/generator besides propelling the automotive vehicle also provides regeneration of power during vehicle braking. The electric motor/generator converts mechanical energy during braking to electrical energy that is then stored in the battery. This is often referred to as regeneration braking. The stored electrical energy in the battery is also used for onboard systems such as air conditioning systems, lighting systems, audio/visual systems, and other electrical systems.
Several fuel cell energy system designs require a high voltage energy source to startup and shutdown the fuel cell energy system. Fuel cells produce energy but do not provide energy storage. Fuel cell energy systems also require transient load assist during energy load events and energy storage during brake regeneration events that can not be supported by the fuel cell energy system alone. Additionally, in order for EVs to be practical for operator use, heightened requirements have been introduced, which include increased fuel efficiency and quicker power reaction times. Power reaction time refers to the amount of time required for a fuel cell system to react to power demands from an operator or needed for different driving conditions.
Furthermore, current fuel cell propulsion systems do not maximize the fuel cell operating time such that it is during the fuel cell's most efficient operating state. Fuel cells have an optimum operating power range within upper and lower limits. Fuel cells are most efficient at steady state or in other words when producing a constant power output. Fuel cells are not as efficient when operated close to and beyond the upper and lower limits. The time the fuel cell is operating at optimum temperature is also not maximized, which may cause significant energy loss.
Therefore, a need exists to support activities during increased energy load events beyond which the fuel cell is able to handle. Also a need exists to store energy during times when extra energy is available. Both of the above needs should be performed as efficiently as possible as to conserve energy.
Moreover, a need exists to utilize the power generated from the fuel cell energy system during the fuel cell's optimum operating state and temperature. By best utilizing and conserving energy the fuel cell energy system is more efficient and cost effective.